An automotive substrate typically has several layers of paint applied during manufacture. The first coating is generally an electrodeposited primer composition that helps the underlying metal substrate resist corrosion. The electrodeposited layer is baked after application to dry and cure the applied layer. The second layer is a primer composition that helps to form a smooth surface for the subsequent paint applications and can also help to resist chipping damage. The primer layer is applied and then baked to dry and cure the layer. The primed substrate then has one or more layers of a basecoat composition applied to give the substrate its final color position. The basecoat composition is generally flash dried, but not cured, to remove a portion of the solvent prior to application of a clearcoat composition. One or more layers of a clearcoat composition is applied, and then the combined basecoat layer(s) and clearcoat compositions are then cured to form a durable aesthetically pleasing finish.
The above described method is generally called a wet-on-wet process, as the clearcoat is applied over the “wet” layer(s) of basecoat.
The phrase “wet-on-wet” means that a layer of a coating composition has not been cured prior to the application of the next layer of coating composition, although some of the solvent may have been removed in a flash-dry step.
Automobile manufacturers typically apply the various layers of coating composition in this manner to obtain the highest levels of appearance and durability for a vehicle finish. However, it is desired to remove at least a portion of the high energy baking steps from the painting process. In order to remove at least a portion of the baking steps, the various layers of coating compositions must be formulated in such a way so as to resist intermixing (sometimes called holdout) with a subsequently applied layer. Intermixing of the applied layers of coating compositions can cause a decrease in the durability and appearance values of the finish.
It is desired to remove the baking step used to cure the primer composition. Removing the primer bake step results in a process that is typically called a wet-on-wet-on-wet or 3-wet process. In this process, one or more layers of a primer composition is applied to the baked electrocoat composition followed by one or more layers of the basecoat composition followed by one or more layers of a clearcoat composition. Optionally, any or all of the applied layers may be flash-dried to remove at least a portion of the solvent. Finally, the primer, basecoat and clearcoat compositions are baked to cure each layer.
The following disclosure provides for waterborne primer, basecoat and clearcoat compositions that can be applied one after another without a baking step in between the application steps.